In conventional printed circuit boards, through-holes serve not only for maintaining electric conduction between printed patterns on both sides of the board, but also for fixing electric and/or electronic elements to the board, in which pins of the elements are inserted into these through-holes and connected to the through-holes by soldering. Accordingly, these through-holes generally have a diameter as large as several millimeters (2 to 6 mm), and the width of a conductor layer portion formed around each of the through-holes on the surface of the board (which conductive layer portion is hereinafter referred to as "land") is usually about 0.5 mm. As a method for producing a printed circuit board having such through-holes, the so-called tenting method using a photocurable resin laminate is known (see Japanese Patent Publication No. 46-3746), and this method has been widely practiced. According to the tenting method, a printed circuit board is produced by laminating a photocurable resin laminate (comprising a support and a photocurable resin layer disposed on one surface of the support) on both surfaces of a metal-clad insulating base board having metallic conductive layers on not only both surfaces thereof but also the entire inner walls of through-holes thereof, so that both surfaces of the metal-clad insulating base board, which are inclusive of both terminal openings of each through-hole, are covered with one surface of the photocurable resin layer remote from the support, exposing the photocurable resin layer on each surface of the metal-clad insulating base board to ultraviolet radiation through a predetermined transmitting pattern to form a photocured resin latent image covering the terminal opening of each through-hole, subjecting the exposed resin layer to development with a liquid developer to form an image of the photocured resin, and etching the metallic conductive layer on each surface of the metal-clad insulating base board in areas other than the image of the photocured resin as an etching resist. The etching resist pattern formed to cover the terminal opening of the through-hole is secured by adhesion to the surface of the land formed around the through-hole, so that the etching solution is prevented from intruding into the through-hole.
Recently, the mounting-on-surface method has been widely adopted as a production technique suitable for increasing the density in printed circuit boards. According to this mounting-on-surface method, an electric and/or electronic element to be mounted on a board is fixed, by soldering, to a conductive layer on the surface of the board. In this case, through-holes are only required to serve as a conductor for producing electric conduction between the circuits on both sides of the board, obviating insertion of pins of elements and, therefore, it has become possible to design a high density printed circuit board having through-holes of a diameter smaller than in the conventional printed circuit boards.
The through-holes having a small diameter are called small-diameter through-holes (or minivia through-holes). The diameter of the small-diameter through-holes is generally not greater than 0.5 mm, and the land width is generally not greater than 100 .mu.m, preferably not greater than 50 .mu.m.
When production of a printed circuit board having small-diameter through-holes is tried by the conventional tenting method, permeation of an etching solution into the small-diameter through-holes is likely to occur, so that it is practically impossible to produce a printed circuit board. In the practical production line, by the thermal expansion and contraction of a negative mask used in the light exposure of the photocurable resin layer and/or by the occurrence of relative dislocation between both the opposite terminal openings of each through-hole which is caused by the formation of bent through-holes due to the vibration of a drill which occurs when through-holes are formed in the insulating base board by drilling, large positional disagreement is caused between the land patterns on the negative mask and the through-holes on the base board, such that the magnitude of this disagreement is larger than the land width. This large positional disagreement causes the outer circumferential edge of the through-hole to partially lack a land, so that the resist pattern is missing in this land-lacking portion, causing permeation of an etching solution into the small-diameter through-hole.
In order to prevent the etching solution from permeating into the small-diameter through-holes, the so-called plugging process has been widely adopted in which a resin called a plugging ink is filled in through-holes and then, a photocurable resin laminate is laminated on a base board.
However, the plugging process is disadvantageous in that since not only is a step of filling the plugging ink, but also a step of removing the plugging ink after the etching operation, is required to be added to the conventional tenting method, and thus, the productivity is drastically reduced.
In these situations, development of a photocurable resin laminate suitable for producing a printed circuit board having small-diameter through-holes, without suffering from permeation of an etching solution into the small-diameter through-holes during the etching step, with very high reliability of conduction of through-holes, and development of a method for producing a printed circuit board by using this photocurable resin laminate, have been strongly desired in the art.
It is therefore an object of the present invention to provide a novel photocurable resin laminate suitable for the production of a printed circuit board having small-diameter through-holes.
Another object of the present invention is to provide a method for producing a printed circuit board having small-diameter through-holes by using this novel photocurable resin laminate.
The foregoing and other objects, features and advantages of the present invention will become apparent from the following detailed description and appended claims taken in connection with the accompanying drawing.